1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image to be visualized by contacting powdery developer (or developing powder) onto a photosensitive member, and in particular to an image forming apparatus such as a copying machine, printer or the like accommodating such a developing device.
2. Description of Related Art
Conventionally, there have been proposed some kinds of developing devices each including a developing roller constituted by a magnet column member with a plurality of magnetic poles and a sleeve overlaid on the outer surface of the magnet column member wherein an electrostatic latent image is turned visual by contacting developing powder retained on the outer surface of the sleeve with the outer surface of a photosensitive member. These developing devices are classified broadly into two types, an inner pole fixed type and an inner pole rotary type. In each of the inner pole fixed type developing devices 101a and 101b as shown in FIGS. 1 and 2 respectively, a magnet column member 103 is non-rotatably fixed and a cylindrical sleeve member 102 overlaid on the outer surface of the magnet column member 103 is rotated in a direction shown by an arrow a. On the other hand, in the inner pole rotary type developing device 101c as shown in FIG. 3, the magnet column member 103 as well as the sleeve member 102 overlaid thereon is rotated in a direction shown by an arrow a'.
Moreover, the inner pole fixed typed developing devices are generally classified into two types, a single pole type as shown in FIG. 1 and a double pole type as shown in FIG. 2. In the single pole typed developing device 101a, as shown in FIG. 1, there is provided a single magnetic pole N confronting to a developing zone Q between the sleeve 102 and the photosensitive member 100 closely facing to each other, and only the developing powder retained by the magnetic pole N is brought into contact with the outer surface of the photosensitive member 100 so as to form a toner image. In the double pole typed developing device 101b, as shown in FIG. 2, there are provided double pairs of magnetic poles (N.sub.1, S.sub.1) and (N.sub.2, S.sub.2) in the outer peripheral portions of the magnet column member 103, wherein the magnetic poles N.sub.1 and N.sub.2 (S.sub.1 and S.sub.2) have the same polarities and the magnetic poles N.sub.1 and N.sub.2 are adjacent to each other in the outer peripheral portion of the magnet column member 103 confronting to the developing zone Q, so that the developing powder portions retained by the magnetic poles N.sub.1 and N.sub.2 are both contacted onto the outer surface of the photosensitive member 100 so as to develop an electrostatic latent image to be visible. It is to be noted here that the transporting speed of the developers in these developing devices of three kinds is set faster than the moving speed of the surface of the photosensitive member.
However, these developing devices 101a, 101b and 101c have various drawbacks respectively as following.